The party was outlawed by the U.S. occupation authorities[why?], but the party organized a network of clandestine cells and was able to obtain a considerable following. It had around 360 000 party members.[2] In 1947 the party initiated armed guerrilla struggle. As the persecution of party intensified, large sections of the party leadership moved to Pyongyang.

The party was opposed to the formation of a South Korean state. In February–March 1948 it instigated general strikes in opposition to the plans to create a separate South Korean state.[3] On April 3, 1948 the party led a popular uprising on Jeju island, against the unilateral declaration of the foundation of the Republic of Korea. In the suppression of the revolt, thousands of islanders were killed (see Jeju massacre).[4]

In one of its first official acts, the South Korean National Assembly passed the National Traitors Act in September 1948, which among other measures, outlawed the Workers' Party of South Korea.[5]